A. Field of the Invention
The invention relates to a method for producing solid particles having a silica coating and for coating solid particles with silica. The method products thus have a core and a shell, whereby the shell has silica or consists of silica, while the core is a solid substance different from silica.
B. Related Art
The inventive method is in principle suitable for the silica coating of any desired solid particles. For example, there can be coated any desired inorganic solids stable under the process conditions of the coating method, in particular acid-labile substances, whereby the method according to the invention focusses on the protection of feature substances. The invention is described in the following with reference to acid-labile feature substances as the solid particles to be coated. The invention, however, is under no circumstances restricted to the coating of acid-labile feature substances with silica.
Documents of value, such as for example bank notes, shares, bonds, deeds, checks and other papers at risk of forgery, such as passports or other identification documents, are normally provided with various security features to increase their falsification security. Such a security feature typically has at least one feature substance. Such feature substances are for example luminescent, magnetic, electrically conductive or in certain wavelength ranges absorbing substances.
However, feature substances often exhibit the disadvantage of low stability toward external influences, such as for example oxygen, moisture, organic solvents and oxidizing or reducing substances. A further, serious problem is the sensitivity of many feature substances to acids.
There is a multiplicity of compounds which per se have physical properties which would render them excellently suitable as feature substances for documents of value. But unfortunately these physical properties, upon the actual employment in a document of value, change more or less quickly, as a result of which the check as to the authenticity of the document of value may lead to incorrect results. The decisive impediment, which makes it impossible for the feature substances to be used in documents of value, often is the sensitivity of the feature substances to acids. Documents of value, and in particular bank notes, very often come in contact with the human skin which has an acidic pH value between 5 and 6. Through the repeated contact with this acidic medium, there occurs in many feature substances a chemical change, which inevitably causes a change in the machine-readable or visually perceptible properties. Therefore, there have already been made attempts to suitably protect substances suitable as feature substances, which, however, cannot meet the high requirements for the resistance of their machine-readable or visually perceptible properties to external influences, in particular to the action of acidic media, which have to be met upon the use in documents of value.
In WO 2006/072380 it is proposed to provide an acid-labile feature substance with a shell consisting substantially of metal oxide. For forming the shell, either one or several feature substances and one or several metal oxide precursors are brought to reaction in a solvent under basic conditions at a pH value higher than 8, or one or several feature substances are dispersed in a solvent under basic conditions at a pH value higher than 8 and then one or several metal oxide precursors dissolved in a solvent are slowly added dropwise, or one or several feature substances and one or several metal oxide precursors are dispersed or dissolved in a solvent at a neutral or slightly basic pH value and then a base is slowly added dropwise. Suitable metal oxide precursors are metal organyl compounds, which through condensation reactions lead to the formation of the metal oxide of the shell. Through the kinetic control of the hydrolysis reaction via parameters such as pH, speed of dropwise addition and temperature there can be prevented an uncontrolled gel formation, which otherwise may lead to undesired agglomerates or extensive three-dimensional networks. As a by-product of the hydrolysis of the metal organyl compounds there may arise volatile organic compounds, which are of disadvantage with respect to the technical process.
From WO 2006/030001 there is known a method for producing a silica coating on particles, in which to a dispersion of the particles one adds simultaneously a solution containing silicate and an acid. In order to prevent agglomeration, the dispersion is acoustically excited, for example, by ultrasound.
Further methods for producing a silica coating on particles are known for example from U.S. Pat. No. 2,885,366, from GB 2 042 574 A and from U.S. Pat. No. 6,132,773.
In the method according to U.S. Pat. No. 2,885,366, by acidifying an alkali metal silicate “active silica” is produced and added to an aqueous suspension of the material to be coated. The acidification is preferably effected with sulphuric acid. In the method disclosed in GB 2 042 574 A in an aqueous dispersion of an inorganic pigment a silicasol is produced, by an alkali metal silicate as well as a compound being added, which compound is able to produce from the alkali metal silicate the silicasol. Such compounds are acid sources such as organic acids, phosphoric acids, borate ester, phosphate ester, alkali metal salts, ammonium salts, boric acid and ethylene carbonate. U.S. Pat. No. 6,132,773 discloses a method for coating particles with a silica shell, in which to the particles to be coated a silicate and an acid source is simultaneously added, whereby for increasing the ionic concentration and thus the silica deposition speed an electrolyte is added. As an acid source for example an inorganic acid, an organic acid or carbonic acid is used, and the electrolyte is an alkali metal salt.
Most of the above-mentioned methods have in common that a dispersion of the solid to be coated is provided, to which both a silicon dioxide precursor and an acid are simultaneously metered. This procedure has various disadvantages. For forming homogeneous, well covering layers with high acid resistance, the quantities of metered silicate and acid must be carefully balanced, i.e. a careful metering control and pH control is required. pH variations lead to irregularities in the silica nanoparticle growth, i.e. to size differences and agglomerations of the silica nanoparticles. In the worst case, the acid-labile feature substance can be dissolved or partially dissolved. Because at the place where the acid is dropwise added there is for a short time, until the intermixture with the surrounding solution, an acid excess, which is sufficient to partially destroy acid-labile features. Furthermore, by-products which arise from the dissolved or partially dissolved feature substance may lead to deviating feature signals, for example to additional luminescence bands in the case of luminous material, which upon authentication checks leads to falsified results, or be toxicologically critical. Furthermore, working with strong acids is not desirable with regard to occupational safety and health.
A comparatively good coating method is the method disclosed in WO 2006/072380 A2. However, it is not based on the acidification of basic silicon dioxide precursors on silicate basis, but on the basic hydrolysis of metalorganic metal oxide precursors. However, this method, too, requires a careful pH control and is susceptible to process instabilities with agglomeration problems resulting therefrom.